An artesian well is a well into which water is forced by pressure under Earth's surface. Artesian wells are possible only under specific geologic conditions. An underground layer of porous sandstone is buried between two layers of rock through which water cannot pass. At some point, water falling as rain or snow passes through cracks in the rock and is trapped in the sandstone. In order to make the well, people drill a narrow hole deep into the sandstone layer. The freed water gushes to the surface like a geyser. The practice of drilling artesian wells is ancient. The Chinese and Egyptians did it, although it took them years. Today's drilling methods make it a much easier and quicker task. In agricultural areas of southern Texas, artesian wells now supply much of the water for irrigation. Nationally, aquifers today provide more than half the water Americans drink.

Read this sentence from the passage--Artesian wells are possible only under specific geologic conditions.

In the passage, this statement is...

a
a commonplace assertion that may or may not be true.
b
an opinion supported by the author's personal exxperience.
c
a factual claim that is supported by scientific facts.
d
a fact that cannot be proven by evidence.